Field of the Invention
The invention relates to a method for preparing R—Fe—B based sintered magnet.
Description of the Related Art
R—Fe—B rare earth sintered magnet has been fast developed and widely applied to the field of the computer hard disk, hybrid power automobile, medical, and wind power generation industries due to its high strength, excellent magnetic properties, and low cost.
Coercivity is a significant index for measuring the magnetic properties of the rare earth sintered magnet, and a typical method for improving the coercivity of the magnet is to add rare earth raw material of pure metal or alloy including Tb or Dy during the melting process. Grain boundary diffusion is a method for diffusing Tb or Dy, which includes melting the grain boundary at high temperature, and diffusing Tb or Dy from the surface along the gain boundary of the magnet to an inner part of the sintered magnet. The method highly improves the utilization rate of the heavy rare earth elements, lowers the usage amount of the heavy rare earth elements, and largely improves the coercivity of the magnet.
Conventional methods for preparing R—Fe—B based sintered magnets includes slurry coating process, vacuum evaporating process, and tumble-plating process.
The slurry coating process includes preparing a slurry including an oxide, fluoride, or oxyfluoride of Tb or Dy, coating the slurry on the surface of the sintered magnet, and placing the coated sintered magnet in a sintering furnace for high temperature treatment and aging treatment after drying, allowing Tb or Dy to enter the inner part of the sintered magnet by gain boundary diffusion. The method has a complicate operation, a large amount of Tb or Dy powder is attached to the magnet piece after treatment, which requires further machining or washing for removal. The process is complicate and easily results in waste. Besides, the slurry coated on the surface of the magnet is still in the form of powder after being dried, thereby being easily falling off, and the increase of the coercivity of the magnet after treatment is not dramatic.
The vacuum evaporating process has high requirement on the evaporation rate of the evaporation source, the evaporation concentration, the temperature, the vacuum degree, and the operating system. However, a distance exists between the magnet to be treated and the evaporation source, so that the space utilization is decreased and the production cost of the treatment is relatively high.
The tumble-plating process includes contacting the rare earth magnet with the diffusion source of the heavy rare earth metal or alloy thereof, and diffusing the heavy rare earth elements to the inner part of the sintered magnet at the high temperature by using tumble-plating like process. Because the diffusion of the heavy rare earth elements to the inner part of the sintered magnet is on the premise that the grain boundary is melted at the high temperature, whereas Pr and Nd in the melted grain boundary are easily replaced by the heavy rare earth elements, so that the sintered magnet and the heavy rare earth elements or alloy are easily stuck together once the movement is not in time, thereby being poorly practical.